1. Field of the Invention
The present invention relates to a thin film bonding method, and more particularly, to a thin film bonding method and a bonding method and apparatus of an optical disk using the same.
2. Description of the Background Art
Recently, following the compact disk (CD) generalized as an optical recording medium, a digital versatile disk (DVD) has been proposed with its standardized size, and various products such as a DVD-ROM, a DVD-RAM, a DVD-R, a DVD-RW and a DVD+RW are becoming common.
Such DVD has a remarkably improved recording density compared to the existing CD and is expected to be widely spread.
For example, the capacity of the DVD is about 4.7 Giga bytes (GB) on the basis of a single-sided single layer, storing a movie of about 2 hours in a VHS class.
Among the optical recording mediums, as shown in FIG. 1, a reproduction-exclusive optical recording medium includes a first substrate 2 having an information record layer with a pit pattern and a reflection film, and a second substrate 6 attached to the first substrate 2 through an adhesive agent 4 applied at an upper portion of the first substrate 2.
The first substrate 2 is a light transmission layer, typically made of a co-polymer substance such as a polycarbonate, and has a thickness of 0.6 mm.
One side of the first substrate 2 is used as an information record layer for recording information by forming the pit pattern, a guide groove, or the like, and the reflection film is formed on the information record layer to reflect a laser beam (LB) made incident through the first substrate 2.
As a dummy substrate, the second substrate 6 is made of the same material as that of the first substrate 2, serving as a protection layer for preventing deformation of the first substrate 2 and a degradation of the reflection film.
The second substrate 6 also has the thickness of about 0.6 mm the same as the first substrate 2 and bonded onto the reflection film of the first substrate 2 by an adhesive agent 4.
Accordingly, the optical recording medium, having the structure that the second substrate 6 serving as a protection layer and the first substrate 2 having information are bonded, have the thickness of 1.2 mm, and formed as a disk type having the diameter of 12 cm.
In the optical recording medium, in order to record information or read information from the medium, a laser beam (LB) generated from an optical pick-up (not shown) is transmitted through the first substrate 2 and irradiated on the information record layer.
In such a case, a size of a beam spot formed on the information record layer determining a record density of the optical recording medium is in proportion to a wavelength of a corresponding light source contained in the optical pick-up and in inverse proportion to a numerical aperture (NA) of the objective lens.
Accordingly, in order to improve a record density of the optical recording medium, it is essential to use a light source having a short wavelength with respect to the thickness of the first substrate 2 and an objective lens having a large numerical aperture.
However, in case of increasing the numerical aperture of the objective lens, since a coma aberration of the optical disk to a tilt is drastically increased in proportion to the third power (NA3) of the numerical aperture, a reliability of the recorded optical information is degraded.
In addition, as shown in FIGS. 2A and 2B, the coma aberration is increased in proportion to the fourth power (t4) of the thickness of transparent substrates 10 and 12 transmitting the light.
The coma aberration will now be described in detail with reference to FIGS. 2A and 2B.
As shown in FIGS. 2A and 2B, when the laser beam (LB) focussed by the objective lens (OL) passes through a first substrate 10 (having a first thickness) of FIG. 2A and a first substrate 12 (having a second thickness) of FIG. 2B, respectively, a tilt angle (xcex81) and an angle (xcex82) due to the laser beam (LB) refracted inside the substrates are formed. If the substrates 10 and 12 are made of the same material, have the same tilt angle (xcex81), and have the same thickness (i.e., the first thickness=the second thickness), the angles (xcex82) refracted inside the substrates 10 and 12 will also be formed to be the same, so that a focus for each substrate 10 and 12 is formed at the same position. However, if the substrates 10 and 12 have different thickness (i.e., the first and the second thickness are different), the position (f) of the focus forming an image varies depending on the thickness of the substrates 10 and 12.
Accordingly, as for the laser beam (LB) sensitive to the thickness of its transmitting substrate has a greater position deflection (xcex41 ) when it transmits the first substrate 10 than a position deflection (xcex42) when it transmits the tilted thin first substrate 12.
In this manner, if the irradiation position of the laser beam (LB) is deviated from a normal position after passing the first substrate 12, a bad effect is made on a focusing control or a tracking control, resulting in that a critical error is generated in recording or reproducing information.
In order to prevent the problems, for an optical recording medium having a recording capacity of higher than about 20 GB, an objective lens having a high numerical aperture and an optical recording medium having a thin light transmission layer substrate has been proposed.
In the proposal, as shown in FIG. 3, the optical disk includes a first substrate 14 forming a protection layer and a thin second substrate 18 bonded on the first substrate 14 by means of an adhesive agent 16 and having an information record layer formed with a pit pattern and a reflection film.
In the construction, the second substrate 18 transmitting the laser beam (LB) has the thickness of 0.1 mm in order to minimize dependence on the substrate thickness of the coma aberration.
The first substrate 14 bonded onto the second substrate 18 through the adhesive agent 16 is a protection layer and has a thickness of 1.1 mm for a compatibility with the existing optical disks having the thickness of 1.2 mm.
However, even though the optical disk having the thin second substrate 18 as a light transmission layer can minimize generation of the coma aberration, since the second substrate 18 is too thin to be put to a practical use with its poor productivity. The reason is because it is not easy to bond the extremely thin second substrate 18 onto the first substrate 14.
The bonding method of the optical disk as shown in FIG. 3 will now be described with reference to FIGS. 4 and 5.
First, in a first step (S20), the first substrate 14 is mounted on a disk type optical disk support member 15 combined at an upper portion of a rotational shaft 17 included in an optical disk bonding apparatus 19.
In a second step (S22), the first substrate 14 mounted on the optical disk support member 15 is rotated, on which a liquid adhesive agent hardened by ultraviolet rays is applied to form a layer of adhesive agent 16.
In a third step (S24), the thin second substrate 18 having a reflection film is mounted on the adhesive agent layer 16.
In a fourth step (S26), pressure is applied at one time onto the entire surface of the second substrate 18, and due to the weight of the second substrate 18, the ultraviolet hardening resin 16 spreads relatively uniformly at the interface of the first substrate 14 and the second substrate 18.
In a fifth step (S28), ultraviolet rays are irradiated to harden the adhesive agent layer 16.
However, in the conventional optical disk bonding method as described above, since the entire surface of the second substrate 18 is pressurized at one time, air is trapped between the first substrate 14 and the second substrate 18 or the thickness of the adhesive agent layer 16 is uneven.
Large air bubbles due to the air trapped between the first substrate 14 and the second substrate 18 degrades the flatness of the optical disk, that is, one of a record density enhancement conditions of the optical disk, and small air bubbles which form due to a temperature change also worsens the flatness of the optical disk.
The flatness degradation of the optical disk causes the optical disk to be bent, a tilt or a vibration of the optical disk or dispersion of a light spot, making a bad influence on the operation of the recording and reproducing information.
Therefore, an object of the present invention is to provide a thin film bonding method that is capable of uniformly bonding a thin film.
Another object of the present invention is to provide a bonding method and apparatus of an optical disk that are capable of bonding thin film with a uniform thickness without an air trap.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a thin film bonding method for bonding a thin film to a target surface by using an adhesive agent, including the steps of:
(A) applying the adhesive agent onto the target surface and placing the thin film onto the target surface;
(B) applying a fluid pressure onto the target surface and the thin film from a central portion to an circumference thereof, so as to gradually proceed the bonding of the thin film and the target surface with the lapse of the time; and
(C) hardening the adhesive agent.
To achieve the above objects, there is further provided a bonding method of an optical disk for bonding a thin film on a first substrate, including the steps of:
(A) placing a thin film on the first substrate with an adhesive agent therebetween;
(B) rotating the first substrate and a thin film while applying a fluid pressure to the first substrate and a thin film according to lapse of time from the central portion to the circumference thereof to allow bonding between the first substrate and a thin film to proceed in a spiral direction thereof; and
(C) hardening the adhesive agent.
To achieve the above objects, there is further provided a bonding apparatus of the optical disk including: a rotational shaft integrally coupled to a drive motor generating a rotational force; a disk support member coupled to one side of the rotational shaft, on which a first substrate is mounted; a first nozzle positioned at a portion of the upper side of the disk support member to supply an adhesive agent onto an upper surface of the first substrate mounted at the disk support member; a pressure supplying means for applying a fluid pressure to the first substrate and a thin film mounted on the first substrate with the adhesive agent therebetween according to lapse of time from the central portion to its circumference to allow bonding between the first substrate and a thin film to proceed in a spiral; and irradiation means for irradiating ultraviolet rays to harden the adhesive agent applied between the first substrate and the thin film.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.